Investigating the role of pumice and metakaolin in the shrinkage and durability performance of concrete

Internal curing is an established strategy for mitigating autogenous shrinkage and extending the service life of low-water-to-binder cementitious materials. However, a comprehensive understanding of the interaction between pumice-based internal curing and metakaolin is still lacking, particularly regarding their combined influence on different shrinkage mechanisms, durability, and mechanical performance. In this study, fine presaturated pumice lightweight aggregates (LWAs) were utilized as an internal curing agent, while metakaolin was incorporated as a supplementary cementitious material (SCM) to evaluate their individual and combined influence on concrete performance. A comprehensive experimental program was conducted on mortar and concrete mixtures with a water-to-binder ratio (w/b) of 0.27 to evaluate rheological behaviour, compressive strength, durability, shrinkage performance, and microstructural characteristics. The incorporation of pumice provided internal curing water that significantly reduced autogenous shrinkage by more than 80% and delayed crack occurrence under restrained shrinkage conditions by approximately 50%, although a moderate increase in drying shrinkage by 20–30% was observed. The addition of metakaolin improved compressive strength, significantly enhanced resistance to chloride ingress and freeze and thaw cycles and reduced drying shrinkage at later ages. Microstructural investigation using scanning electron microscopy (SEM) and image analysis revealed a dense interfacial transition zone (ITZ) around the pumice particles, as well as evidence of enhanced hydration in the surrounding matrix due to the release of internal curing water. The combined use of pumice and metakaolin effectively balanced shrinkage mitigation and durability enhancement, demonstrating their potential for producing durable, crack-resistant high-performance cementitious composites.